Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. Prosthetic heart valves can be used to replace any of these naturally occurring valves. Two primary types of heart valve replacements or prostheses are known. One is a mechanical-type heart valve which uses a pivoting mechanical closure to provide unidirectional blood flow. The other is a tissue-type or "bioprosthetic" valve which is constructed with natural-tissue valve leaflets which function much like a natural human heart valve, imitating the natural action of the flexible heart valve leaflets which seal against each other or coapt between adjacent tissue junctions known as commissures. Each type of prosthetic valve has its own attendant advantages and drawbacks.
Operating much like a rigid mechanical check valve, mechanical heart valves are robust and long lived but require that valve implant patients utilize blood thinners for the rest of their lives to prevent clotting. They also generate a clicking noise when the mechanical closure seats against the associated valve structure at each beat of the heart. In contrast, tissue-type valve leaflets are flexible, silent, and do not require the use of blood thinners. However, naturally occurring processes within the human body may attack and stiffen or "calcify" the tissue leaflets of the valve over time, particularly at high-stress areas of the valve such as at the commissure junctions between the valve leaflets and at the peripheral leaflet attachment points or "cusps" at the outer edge of each leaflet. Further, the valves are subject to stresses from constant mechanical operation within the body. Accordingly, the valves wear out over time and need to be replaced. Tissue-type heart valves are also considerably more difficult and time consuming to manufacture.
Though both mechanical-type and tissue-type heart valves must be manufactured to exacting standards and tolerances in order to function for years within the dynamic environment of a living patient's heart, mechanical-type replacement valves can be mass produced by utilizing mechanized processes and standardized parts. In contrast, tissue-type prosthetic valves are made by hand by highly trained and skilled assembly workers. Typically, tissue-type prosthetic valves are constructed by sewing two or three flexible natural tissue leaflets to a generally circular supporting wire frame or stent. The wire frame or stent is constructed to provide a dimensionally stable support structure for the valve leaflets which imparts a certain degree of controlled flexibility to reduce stress on the leaflet tissue during valve closure. A biocompatible cloth covering on the wire frame or stent provides sewing attachment points for the leaflet commissures and cusps. Similarly, a cloth covered suture ring can be attached to the wire frame or stent to provide an attachment site for sewing the valve structure in position within the patient's heart during a surgical valve replacement procedure.
With over fifteen years of clinical experience supporting their utilization, tissue-type prosthetic heart valves have proven to be an unqualified success. Recently their use has been proposed in conjunction with mechanical artificial hearts and mechanical left ventricular assist devices (LVADs) in order to reduce damage to blood cells and the associated risk of clotting without using blood thinners. Accordingly, a need is developing for a tissue-type prosthetic heart valve that can be adapted for use in conjunction with such mechanical pumping systems. This developing need for adaptability has highlighted one of the drawbacks associated with tissue-type valves--namely, the time consuming and laborious hand-made assembly process. In order to provide consistent, high-quality tissue-type heart valves having stable, functional valve leaflets, highly skilled and highly experienced assembly personnel must meticulously wrap and sew each leaflet and valve component into an approved, dimensionally appropriate valve assembly. Because of variations in tissue thickness, compliance and stitching, each completed valve assembly must be fine tuned using additional hand-crafted techniques to ensure proper coaptation and functional longevity of the valve leaflets. As a result, new challenges are being placed upon the manufacturers of tissue-type prosthetic valves in order to meet the increasing demand and the increasing range of uses for these invaluable devices.
Accordingly, consistent with the developing practice of the medical profession, there is a continuing need for improved tissue-type prosthetic heart valves which incorporate the lessons learned in clinical experience, particularly the reduction of stress on the valve leaflets while maintaining desirable structural and functional features. Additionally, there is a growing need for improved tissue-type prosthetic heart valves which can be adapted for use in a variety of positions within the natural heart or in mechanical pumps, such as artificial hearts or ventricular assist devices, as well as alternative locations in the circulatory system. Further, in order to address growing demand for these devices, there is a need for tissue-type heart valves that are simpler and easier to manufacture in a more consistent manner than are existing valves.